1. Field of the Invention
This invention relates to rotary machines, and specifically to turbine wheel assemblies of a gas turbine engine.
2. Description of the Prior Art
Modern gas turbine engines comprise the principal class of rotary machines to which the present concepts apply. In a gas turbine engine working medium gases are compressed in a compression section of the engine and are flowed to a combustion section where fuel is mixed with the gases and burned to add energy to the flowing medium. The high energy medium is subsequently flowed to a turbine section where a portion of the energy is extracted and applied to drive the engine compressor.
The turbine includes a plurality alternating rows of rotor blades and stator vanes. Each row of stator vanes directs the working medium gases to a preferred angle of entry into the downstream row of rotor blades. The rotor blades in turn extract energy from the medium gases for driving the engine compressor. Each row of rotor blades is mounted around the periphery of a circular disk structure. The combination of rotor blades and supporting disk are known as a wheel assembly. During operation of a gas turbine engine, wheel rotation speeds in excess of seven thousand five hundred revolutions per minute (7500 rpm) are common.
Each blade of the wheel assembly has a root section which engages a correspondingly contoured attachment slot in the disk. Intricate root geometries have been devised to transfer centripetal restraining loads from the disk to each blade. One widely used geometry is aptly described as a "fir tree" attachment. Such an attachment is illustrated in FIG. 1, and the present invention is later described with respect thereto.
One factor limiting the life of rotor blades and disks is the low cycle fatigue life of the material from which the respective components are fabricated. Each cycle of operation loads the components to a given stress level. After repeated occurrence, each cycled component will ultimately develop fatigue cracks. At very low stress levels the number of cycles before cracks appear is nearly infinite. At high stress levels, the number of cycles is severely limited. The fatigue life of each part is referred to as its LCF life.
The present state of the attachment art as practiced in the most modern engines today is as described in the FIG. 3 (Prior Art) illustration of a portion of a "fir tree" type attachment. The teeth on the blade root extend to engage corresponding grooves in the disk attachment slot. Each groove in the disk is formed to a single radius. Similarly, the teeth in the disk slot extend to engage corresponding grooves in the blade root. Each groove in the root is formed of a single radius. The concepts of the present invention depart from those of the prior art illustration in the manner hereinafter disclosed.